1. The Field of the Invention
The present invention relates to structural braces. More particularly, the present invention relates to structural braces adapted to absorb seismic magnitude forces by undergoing plastic deformation while maintaining the structural integrity of the frame structure.
2. The Relevant Technology
For decades steel frame structures have been a mainstay in the construction of everything from low-rise apartment buildings to enormous skyscrapers dominating modern city sky lines. The strength and versatility of steel is one reason for the lasting popularity of steel as a building material. In recent years, steel frame structures have been the focus of new innovation. Much of this innovation is directed to minimize the effects of earthquakes on steel frame structures. Earthquakes provide a unique challenge to building construction due to the magnitude of the forces that can be exerted on the frame of the building. A variety of building techniques have been utilized to minimize the impact of seismic forces exerted on buildings during an earthquake.
One mechanism that has been developed to minimize the impact of seismic forces on buildings are structural braces that are adapted to absorb seismic energy through plastic deformation. While the braces are adapted to absorb energy by plastic deformation, they are also configured to resist buckling. While several embodiments of these energy absorbing braces exist, one popular design incorporates a steel core and a concrete filled bracing element. The steel core includes a yielding portion adapted to undergo plastic deformation when subjected to seismic magnitude forces. Compressive and/or tensile forces experienced during an earthquake are absorbed by compression or elongation of the steel core. While the strength of the steel core will decrease as a result of buckling, the concrete filled bracing element provides the required rigidity to limit this buckling to allow the structural brace to provide structural support. In short, the steel core is adapted to dissipate seismic energy while the concrete filled bracing element is adapted to maintain the integrity of the structural brace when the steel core is deformed. The use of energy absorbing braces allows a building to absorb the seismic energy experienced during an earthquake. This permits buildings to be designed and manufactured with lighter, less massive, and less expensive structural members while maintaining the building's ability to withstand forces produced during an earthquake.
Energy absorbing braces provide a functional aspect that is often independent of aesthetic or architectural details of the building. For example, the seismic load to be absorbed by a brace can dictate brace dimensions that are contrary to a span desired for the building's architecture. This is particularly problematic where the dimensions of the brace, as dictated by the seismic load to be carried, are much larger and/or longer than the frame dimensions where the brace is to be positioned. The conflict of design elements and seismic load can be a seemingly irretractable problem in existing architecture. This is because such seismic loads were not often considered in the design of older buildings. Due to the demand for seismic retrofitting of existing structures, the challenges presented by the interplay of design details and seismic needs can make a seismic retrofitting of an existing building either impractical or overly expensive.
The seismic load capacity of bearing braces can also be affected where the architectural details of the building dictate the dimensions of the bearing brace rather than seismic factors. The load capacity of a bearing brace is dictated by a variety of factors including the length and cross-sectional area of the core member undergoing plastic deformation. For example, where the bearing brace is of a small length and width to accommodate a smaller span in the building framework, the number and magnitude of cycles that can be experienced during a seismic event without resulting in failure of the brace are substantially limited.